This invention relates to dual wall, air gap engine exhaust conduits, such as exhaust pipes, downpipes, exhaust manifolds and the like, and more particularly to air gap exhaust conduits having superior acoustical properties and excellent thermal operational efficiency.
Exhaust component technology has changed markedly over recent decades, from simple cast exhaust manifolds and ordinary steel exhaust pipe components, to sophisticated, lightweight and durable systems including catalytic converters. Lightweight stainless steel exhaust manifolds are replacing the heavy cast structures. Recent developments have enabled dual wall, air gap, heat durable steel exhaust manifolds to be readily economically produced. This type of manifold includes a thin inner liner and an outer jacket. Exhaust pipe components such as downpipes likewise have been developed with a thin inner liner and a thicker outer jacket. These thin liners heat rapidly from the exhaust gases after engine startup, to achieve quicker "light off" of the downstream catalytic converter. The result is a much more efficient system with emissions that meet federal standards.
However, noise radiation is a drawback to these lightweight systems. Hence, efforts have been made heretofore to modify these systems for improving sound quality and reducing sound intensity. Typically, the prior art mechanisms use friction to reduce noise. Frictional contact between the components, however, also involves the creation of a heat sink, i.e., thermal inertia, which detracts significantly from the quick temperature rise needed in the structure for emissions control.
Another noise control technique which was suggested years ago was to form an air gap to reduce noise and have one or both walls of a double wall conduit crimped to form surface deformations which served to hold one wall partially spaced from the other by engaging the other wall with the undulations or crimp areas so formed, as in U.S. Pat. No. 3,133,612. However, these very deformations create substantial physical contact and thermal exchange between the inner and outer walls along the length thereof. That was acceptable in the early 1960's, but today's environmental requirements demand that the catalytic converter light off shortly after engine startup, which necessitates low heat energy absorption in the inner liner without creating a heat sink of the type that would result from the structures in that patent.
It has been determined that the emissions standards can be met if the liner is spaced from the jacket everywhere except at the inlet ends of the liner and jacket, and at the outlet of the liner, while also achieving improved sound characteristics. Crimping or indentations in the outer jacket which would engage the liner along the length of the structure, so as to create a heat sink and thereby heat inertia, are to be avoided, since that severely detracts from the thermal efficiency of the liner structure.
The applicants herein have discovered an engine exhaust conduit structure that employs indentations in the jacket in a manner which does not hinder the thermal efficiency of the air gap, dual wall structure, and significantly improves sound quality.